The state of art knows single action semiautomatic pistols such as the Colt 1911 gun, whose hammer can be cocked by the thumb or by sliding backward the breechblock either manually or due to a shot. In both cases, the hammer always stands in the same position, somewhat backwards, with it being necessary only to exert some pressure of the forefinger on the trigger to fire off.
Guns of this kind can be locked with the hammer in the cocked position. The only way to uncock the gun after its unlocking is to thumb the hammer and lead it down, little by little, after pulling the trigger.
The risk of accidental shots due to trigger sensitivity when the gun is unlocked requires it being carried inside a holster always locked or with the hammer down and unlocked. In both cases, a sudden draw out of the pistol requires a conscious act to either unlock or to cock it with the thumb.
For this reason, double action pistols were developed. These pistols, when carried unlocked and in perfect safety conditions, can fire the first shot just by pulling the trigger which, in a long stroke, such as with revolvers, cocks the hammer, releasing it further for shooting in a continuous motion. After the first shot, the hammer stands cocked like with single action pistols.
Nowadays, there are four main kinds of safety devices for double action pistols, among which there are some which afford or help afford a safe hammer uncocking. The oldest one is a safety lever assembled in the sliding breechblock, which nowadays appears in WHALTER mod. P-38, PPK, PP and SMITH & WESSON cal. 9 mm and 0.45 pistols, and in RUGER mod. P-85 and BERETA mod. 92 weapons. In all these models, when the latch bolt is turned down to uncock the hammer, it remains down positioned, requiring a conscious turning up double action shooting. An exception is made with regard to the STEIR GB 9 mm pistol whose latch, when lowered to uncock the hammer, safely turns back automatically to the unlock position to allow immediate double action.
Similar devices can be found in pistols such as the HECKLER & KOCH P9 and STAR M 30, which have a safety lever assembled in the sliding breechblock. When activated, the safety lever clocks the firing pin but does not uncock the hammer, which requires further pulling of the trigger. This device requires the pulling of the trigger, after duly setting the locking lever to the lock position, in order to uncock the hammer without shooting. Additionally, it is necessary to unlock the gun for further double action shooting.
Such locking devices have the inconvenience of being assembled in the sliding breechblock, making difficult operation with the thumb, and making possible an unintentional action, when operating manually over the grip to hammer cocking, to tuck the pistol in a narrow holster or to pull it out of the helster.
Another device is the uncocking lever, appearing in the SIG-SAUER P-226 pistols. This device is not properly a safety device, but rather a lever located in the front grip, which, when lowered with the trigger pulled, safely uncocks the hammer. After that, the lever turns up automatically to the initial position, allowing double action shooting.
Another device employs a lock assembled into the frame that just blocks the sear. It can be applied only when the trigger is pulled, and requires that the hammer be lowered through manual control, while pulling the trigger for uncocking. This device appears in the BERETA 92 S, BENELLI, BERNARDELLI, CZ-75 and TAURUS PT 99 and 92, among others.
This device allows one to carry the above pistols cocked and locked, in the way some self defense experts recommend based on the COLT 1911. However, the double action advantage is jeopardized when the gun is set to operate in this condition due to the dangerous uncocking required.